Semiconductor materials contain majority and minority carriers that govern the performance of transistors and other semiconductor devices fabricated from such materials. In the design and fabrication of such devices, it is important to know the concentration of these carriers at various depths within the semiconductor material.
It is known in the art to measure carrier concentration in semiconductor materials using electrochemical profiling techniques, also known as anodic dissolution techniques. According to these techniques, concentration measurements are made at various levels in a semiconductor material as the semiconductor material is controllably dissolved, layer-by-layer.
For example, U.S. Pat. No. 4,028,207 issued in 1977 to M. M. Faktor, et al. discloses a measuring apparatus wherein an electrolyte forms a Schottky barrier with the semiconductor surface while controllably dissolving the semiconductor surface to expose new regions. When high anodic potential is applied, the electrolyte etches more deeply into the semiconductor, and data relating carrier concentration to the new depth may be obtained. The Semilab MCS-90, manufactured by Semiconductor Physics Laboratory RT of Budapest, Hungary, is an example of a modern electrochemical profiler.
FIG. 1 depicts a typical apparatus 2 for making electrochemical profile measurements according to the prior art, an apparatus with which the present method may be practiced to provide improved measurement data. The semiconductor specimen 4 is supported on an electrolytic cell 6 that typically includes a saturated calomel reference electrode ("SCE" ) 8, a graphite cathode auxiliary electrode 10, and a light source 12 (used to create holes when specimen 4 is n-type material).
A suitable mechanism 14 exerts force against specimen 4 to maintain intimate contact between the specimen surface 16 that is to be etched, and an area (A) of electrolyte 18. The interface at surface 16 between the specimen 4 and the electrolyte 18 forms a rectifying or so-called Schottky barrier. Collectively, the semiconductor-electrolyte system including interface surface 16 forms what is commonly referred to as the semiconductor working electrode. Preferably a gold electrode 20 contacts surface 16, while contact mechanism 40 urges ohmic contact between platinum pins 22 and 24 and the non-etched side 26 of the specimen 4. A pump 28 is optionally used in the prior art, generally to circulate the electrolyte 18.
A bias voltage source 30 provides a swept anodic potential V.sub.a (t) that is coupled to the first input of a potentiostat 32, whose second input is coupled to the SCE electrode 8. Potentiostat 32 provides an anodic output etching current i(t) that is coupled to the graphite electrode 10. Potentiostat 32 essentially imposes the reference, or anodic, potential V.sub.a (t) upon the SCE electrode 8. As used herein, the anodic potential V.sub.a (t) is understood to be referenced to the SCE electrode 8.
The platinum pins 22, 24 couple a signal generator 34 to specimen 4 to facilitate data measurements using the gold electrode 20 and alternating current ("AC") circuitry 38. As the anodic voltage from source 30 is swept (e.g., varied), the semiconductor depletion region at the semiconductor interface 16 changes, thus varying the interface admittance. The admittance at the specimen-electrolyte interface 16 has real and imaginary components, whose respective magnitudes may be determined by AC circuitry 38.
AC circuitry 38 typically includes phase-sensitive components that permit characterization of specimen 4 at various depths, using, for example, the real and imaginary interface admittance components to determine the interface capacitance C.sub.Si-E. Direct current ("DC") circuitry 36 typically includes components for integrating the anodic current i(t) to enable a determination of the semiconductor etch depth W.sub.R. Circuitry 36 and 38 can provide output data that may, for example, be used to represent semiconductor parameters as a function of etch depth, in an X-Y plot, for example.
In practice, for an n-type semiconductor specimen, anodic dissolution of surface 16 typically occurs at a fixed anodic potential V.sub.a that is maintained by the potentiostat 32, at a rate determined by the number of minority carriers (holes) produced by photons from light source 12. For p-type material, sufficient anodic current i(t) exists without illumination from light source 12, due to the relatively large number of available minority carriers (electrons).
As understood by those skilled in the relevant art, data representing semiconductor depth (W.sub.R) are provided by taking the integral of the dissolution current i(t), ##EQU1## where M is the semiconductor's molecular weight, N is the effective dissolution valence of the semiconductor specimen, F is the Faraday constant, D is the semiconductor density, and A is the dissolution area.
To produce a carrier concentration profile, W.sub.R must be added to the depletion width W.sub.D, ##EQU2## where, as noted, A is the dissolution or etch area, and C.sub.Si-E is the overall interface capacitance.
From the foregoing, it will be appreciated that if the effective dissolution valence N cannot be reliably and consistently maintained and determined, meaningful measurements of W.sub.R cannot be attained. Similarly, one cannot obtain accurate and reliable measurements of other semiconductor parameters that depend upon N or W.sub.R.
While electrochemical profile measurements as described above have been used for many years, in practice the data obtained can be inconsistent and vary widely. In addition to the measurements being dependent upon the electrolyte selected and the electrolyte pH, the depth calculations are very sensitive to the effective dissolution valence of the semiconductor (the number of electronic charges transferred per atom of semiconductor dissolved). In fact, the inability to reliably maintain a consistent effective dissolution valence has caused electrochemical depth profiling to fall out of favor for silicon specimens, especially for multilayer silicon specimens.
What is needed in electrochemical depth profiling is a method for reliably maintaining a consistent effective dissolution valence for a semiconductor specimen, thereby promoting more accurate depth measurement data. The present invention provides such a method.